The present invention relates in general to liquid crystal display devices; and, more particularly, the invention relates to a liquid crystal display device of the type having a transmissive display region or area and a reflective display area in every picture element, and also having two-mode display functions, including a transmission mode and a reflection mode.
Liquid crystal display devices offer several features, such as slim size and light weight, plus low power consumption, and thus they are widely employed as image/text data display devices in a variety of types of information equipment typically including personal computers (PCs), and they are also used in portable or mobile information terminals, digital cameras and camera-embedded video tape recorders (VTRs). In recent years, the demand for low power consumption is becoming stronger in accordance with rapid popularization of small-size digital equipment, such as, in particular, handheld wireless telephone handsets, known as cellular phones, and mobile information terminals and the like.
Most liquid crystal display devices for use in personal computers and liquid crystal display monitors are generally of the type employing a transmission-type display mode (simply referred to as a transmission mode hereinafter) which visualizes images by controlling the transmission amount of light emitted from an illumination device—this is typically installed on the back face of a liquid crystal display panel and is called a back-light-on the liquid crystal display panel side. However, the electrical power being consumed by the backlight amounts for more than 50 percent (%) of the power that is consumed by the entire liquid crystal display device. Due to this, in battery-driven small-size equipment, such as cellular handy phones and mobile information terminals and the like, the length of the operating time thereof is significantly limited.
In view of this, these small-size electronic devices are often designed to employ liquid crystal display devices of the type using a reflection-type display mode (simply referred to hereafter as a reflection mode) which visualizes images by permitting reflection of ambient light without the use of any backlight. Such reflection of ambient light is achievable by providing a reflective plate on the outer surface or inner surface of a liquid crystal display panel, or alternatively by adding reflecting functions to the pixel-forming electrodes.
The reflection mode liquid crystal display devices are faced with a problem that the visual recognizability or viewability of an on-screen display decreases when the ambient light is less in intensity (i.e. in the dark). For equipment such as cellular phones that are being used everywhere without regard to indoor and outdoor environments, it is necessary to offer required usability irrespective of whether the ambient light is bright or dark. As a remedy for this, a liquid crystal display device of the partial-transmission/partial-reflection type has been proposed, which device has both transmission-mode display functionality, and reflection-mode display functionality with a transmission mode display area and a reflection mode display area being formed in the region of a single picture element or “pixel” (see JP-A-2000-19563).
Unfortunately, in the transmission mode display and the reflection mode display, there are different optimal values for the voltages which are applied between a pixel electrode and an opposite or “counter” electrode in order to create an appropriate electric field in a liquid crystal layer. The optimal voltage as stated herein refers to a voltage which is provided at the time of obtaining flicker-free on-screen images. In most cases, an optimal voltage in the transmission mode is lower in potential than that in the reflection mode.
In the prior known partial-transmissive/reflective liquid crystal display device referred to above, the voltage being applied to the counter electrode in order to form an electric field between it and the counter electrode is simply set to the same level in any display mode, i.e. of either the transmission mode or the reflection mode. Accordingly, even when onscreen images are of good quality in the transmission mode, flicker can occur on the display screen when the display mode is switched to the reflection mode, resulting in a decrease in display image quality.
It is therefore an object of the present invention to provide a liquid crystal display device of the partial-transmission/reflection type having displaying functions in both a transmission mode and a reflection mode, which device is capable of improving the image quality in each display mode to thereby obtain high-quality on-screen display images in either display mode.